<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div class="parent">
        <div class="test"></div>
    </div>
</body>
</html>
<style>
.parent{
    border: 2px solid #333;
    width: 100px;
    height: 100px;

    /* 3 */
     /* display: flex;
     justify-content: center;
     align-items: center; */

     position: relative;

    
}
.test{
    width: 40px;
    height: 40px;
    background-color: pink;
    /* 1 */
    /* position: absolute; 
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%); */

    /* 2 */
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    margin: auto;   
}
</style>